The present invention relates to a waste heat recovery apparatus such as economizer and heat pipe type air preheater for recovering waste heat from exhaust gas (below 400.degree. C.) discharged from heavy oil fired boilers and the like.
Conventionally, in recovering waste heat from exhaust gas, which results from the combustion of heavy oil, by means of a heat exchanger such as economizer, the gas is introduced into a casing containing a set of heat exchanger tubes, and water is fed into the tubes so that it is heated by the gas. Thereupon, the heat exchanger tubes may be corroded by sulfuric acid contained in the exhaust gas, or be coated with dust to cause an exhaust gas passage to be closed. The inventor of the present application considered a waste heat recovery apparatus comprising a high-temperature-side casing unit (hereinafter referred to as high-temperature unit) containing a set of heat exchanger tubes, a low-temperature-side casing unit (hereinafter referred to as low-temperature unit) containing another set of heat exchanger tubes, and an intermediate duct connecting the two units. In cooling exhaust gas from about 200.degree. C. to about 100.degree. C. by a heat exchanger and then discharging it into the outside air, the gas of about 145.degree. to 200.degree. C. is introduced into the high-temperature unit, while the gas of about 100.degree. to 140.degree. C. is passed into the low-temperature unit. In order to remove dust on the heat exchanger tubes, in the apparatus of this type, a dry-type cleaner or dust cleaning device and a flush-type cleaner or dust cleaning device are used in the high- and low-temperature units, respectively. The apparatus is subject to drawbacks as follows. When the heat exchanger tubes are showered frontally and from above by means of a spraying tube of the flush-type cleaner, flushing water comes into contact with the exhaust gas, casing, and heat exchanger tubes on the low-temperature unit side to produce steam. The steam fills the intermediate duct, moistening dust on the duct and the heat exchanger tubes on the rear side of the high-temperature unit. This is caused because the exhaust gas flow is temporarily retarded by the flushing water, allowing the steam to flow back into the high-temperature unit. As a result, in the dry-type cleaner of the high-temperature unit, the sweep resistance of cleaner blades is increased by the wet dust on the heat exchanger tubes. Thus, the blades are deformed and disabled from effecting a satisfactory cleaning. The dust on the heat exchanger tubes accumulates to close the gas passage and corrode the casing of the high-temperature unit.
In general, the casing of the high-temperature unit may be formed of stainless steel or sulfuric-acid-resisting steel; the casing of the low-temperature unit of a steel plate with lead-plate lining in contact surfaces of exhasut gas or FRP (fiberglass reinforced plastic), and the intermediate duct of a steel plate with lead-plate lining in contact surfaces of exhaust gas, FRP or stainless steel.
As another problem, in boilers used in cold districts, if a stainless steel stack is used without a smoke desulfurization system, the temperature of exhaust gas discharged into the air is kept at more than about 160.degree. C. at its exhaust port to prevent corrosion at the dew point of sulfuric acid. If the inlet-temperature of exhaust gas is lower by low load of boiler in summer, therefore, the waste heat recovery apparatus cannot be operated due to a lower outlet-temperature of exhaust gas.